1. Field of the Invention
The present invention relates to an ignition coil which is used mainly in an internal combustion engine for a vehicle.
2. Description of the Related Art
The ignition coil includes a transformer which is formed by arranging a primary coil, a secondary coil outside the primary coil, and a center core inside the primary coil. A side core provided outside the center core constitutes a magnetic circuit in combination with the center core. After these components have been contained in a case and connected to interior parts of a vehicle by means of a connector assembly for connection with a vehicle harness, they are fixed and insulated by an insulating resin. Generally, electromagnetic steel plates are employed for making the center core and the side core, and thermosetting resin such as epoxy resin is employed as the insulating resin. The case is provided with an output terminal, which is connected to an ignition plug of an internal combustion engine by way of a high voltage cord.
In the ignition coil so constructed as described above, supply and disconnection of a primary current passed through the primary coil is controlled by control signals from a control unit in the internal combustion engine, by way of the connector assembly. When the primary current passed through the primary coil is cut off by the control signal at a determined ignition timing of the internal combustion engine, a counter electromotive force is generated in the primary coil, and high electric voltage is generated in the secondary coil. The generated high voltage is inputted to the ignition plug of the internal combustion engine which is connected by way of the high voltage cord.
[Patent Document 1] Japanese Patent Publication No. JP-A-6-112069
The side core formed of the electromagnetic steel plate exists in the insulating resin which has a different coefficient of linear expansion, and distortion may occur in the insulating resin when a thermal stress has been applied. In case where the thermal stresses have been repeatedly applied, an abrasion or a crack may occur in the insulating resin. Under the circumstances, the crack of the insulating resin starting from the side core has been prevented by mounting a core cover on the side core. However, the crack in the insulating resin which has occurred in the side core may exert bad influence on those components which are faced with the side core. For example, there is such probability that distortion may occur in the secondary coil (especially, in its bobbin), around the bobbin of the secondary coil which is located at a position close to the side core, and formed of material having high adherence to the insulating resin. The abrasion or crack which has occurred as the results will be a cause of an internal leak, and the distortion will be a cause of a break of the coil. Therefore, it has been a problem that this may lead to such a phenomenon that output voltage of the ignition device for the internal combustion engine is lowered or not at all generated.